Making LayoutUnit conversions to Int types explicit in core/accessibility

LayoutUnit misuse, often from improper conversion to other types, is a 
constant source of bugs in Blink. 
These conversion sites were discovered by toggling the int conversions to 
explicit and finding the compiler errors. 

BUG=350474
TEST=none

Review URL: https://codereview.chromium.org/248963002

git-svn-id: svn://svn.chromium.org/blink/trunk@172455 bbb929c8-8fbe-4397-9dbb-9b2b20218538